Mechanical probe heads in use to date on coordinate-measuring instruments can, in principle, be classified into two categories, namely probe heads of the so-called measuring type and probe heads of the so-called switching type. Probe heads of the measuring type supply an analog signal which is proportional to probe-pin deflection. Probe heads of the switching type provide a precision seat or bearing which determines the at-rest or null position of a deflectable probe pin when out of workpiece contact, and they supply a pulse-like electrical signal only at the moment of workpiece contact, namely, when the probe pin lifts off its seat, in the course of a workpiece-contacting procedure. Such probe heads are illustratively described in U.S. Pat. No. 4,153,998.
The probe pin is mounted to a movable part of the probe head, and a compression spring is conventionally used to preload the movable part into the seated at-rest or null position in the bearing. More refined probe heads furthermore have a so-called taring device by which the force of the compression spring can be adjusted, in compensation for gravitational effects. In this way, the contacting force which the probe pin exerts on the workpiece to be measured can be preselected, and the contact force can be maintained constant even if probe pins of different weight are substituted in the probe head.
However, there are disadvantages in using a compression spring in the pre-loading of a probe pin. On the one hand, during the lifting movement and in the course of the contacting process, as well as in making the taring adjustment, the compression spring produces torsional forces which can so twist the probe pin that it may not return to its seated at-rest position in the bearing. To prevent such an event, additional structural measures are necessary. Still further, deflection force increases during the deflection procedure, as a function of stiffness of the compression spring, and this circumstance presents the danger of breaking a relatively thin probe pin of hard metal or ceramic material.
Finally, long compression springs, as are required for large deflection paths, tend to kink, and resulting transverse forces impair the precision of reproducing the null position of the probe pin, upon return to its seated position in the bearing.
Federal Republic of Germany OS 3,420,139 discloses a sWitching-type probe head having a housing within which compressed air is operative. In this probe head, air pressure contributes to loading the probe pin, which is carried by a normally seated valve member; however, the actual pre-loading member in the known probe head is still a spring. The pressure within the probe head is not adjustable and is used solely to recognize the moment of workpiece contact during a contacting procedure.
Federal Republic of Germany OS 2,535,249 describes a probe head of the measuring type, having a probe pin which is displaceable linearly in all three directions in space, along three orthogonal guide systems. Plural pressure-fluid cylinders are contained in this probe head; they are arranged in each case parallel to the guide systems and are provided to assure workpiece contact by the probe pin, with constant contacting force. Aside from the fact that this is a probe pin of a different type, the pressure in the pressure-fluid cylinders cannot be changed, so that it is not possible to establish different contacting forces or to tare the probe head as a function of the weight of a given probe pin.
In the art of coordinate measurement, the trend is now toward making instruments which are faster and more variable, in order to be able to carry out a large number of different measuring tasks with a single instrument. As far as the probe head is concerned, this means that the contacting force which the probe pin exerts on the workpiece is to be changeable as rapidly as possible during a measuring procedure. Furthermore, when a given measurement program calls for use of many different probe pins at different times in the course of the program, the taring to effect gravitational compensation for each newly mounted probe pin must be performed as rapidly as possible, so that no measurement time is lost. This cannot be achieved with the necessary speed when a spring is the pre-loading means, even if its spring force were to be automatically adjustable, as by means of a setting motor.